1. Field of the Invention
The present invention relates to a manufacturing method of valve springs of high strength superior in durability.
2. Description of the Prior Art
As the fatigue strength of valve springs is closely related to residual compression stress on the surface of the valve springs, a method has been developed for applying higher residual compressive stress to the surface of valve springs to a depth of 0.1 to 0.2 mm. In such a conventional method, various kinds of cut wires different in diameter and hardness are shot-blasted to the valve springs at plural steps after high temperature nitriding process thereof. Japanese Patent Laid-open Publication 5(1993)-331535 disclosed a shot peening method of applying hard shot of Hv 650-850 in hardness to a coiled valve spring made of an oil-tempered wire after treatment thereof by low temperature carbo-nitriding process. As a high strength wire material of the valve springs, an oil-tempered wire has been proposed containing 0.45-0.8% C., 1.2-2.5% S1, 0.5-1.5% Mn and 0.5-2.0 Cr, by weight, and at least one metallic element selected from the group of 0.1-0.7 Mo. 0.05-0.6% V, 0.2-2.0% Ni and 0.01-0.2% Nb, by weight and containing Fe and impurity elements as a remainder.
However, when am oil-tempered wire of high strength designated as SWOCN-V is treated by high temperature carbo-nitriding process at about 500.degree. C. the surface hardness of the wire becomes more than Hv 900. and the inner hardness of the wire becomes Hv 570. In the case that a valve spring of such an oil-tempered wire is applied with hard shot peening at plural steps such as two steps or three steps, the fatigue strength of the valve spring Is greatly enhanced. It is, however, necessary required to apply the shot peening to the valve spring for a long period of time (for instance, 1.5-2.5 hours) at plural steps in a tumbling shot machine. The shot peening causes defacement of rubber belts in the tumbling shot machine in a short period of time due to strong shot of hard materials, resulting in unexpected trouble in production of the valve springs. If the time of shot peening was shortened to enhance productivity of the valve springs, a large difference in fatigue strength would occur in each product of the valve springs. To eliminate the difference In fatigue strength in a reliable manner, it is required to reduce the number of valve springs to be applied with the shot peening. This results in reduction of productivity of the valve springs, If the speed of shot peening is lowered to reduce damage to the tumbling shot machine, the residual stress decreases in depth, resulting in a decrease of the internal fatigue strength of the valve springs. Furthermore, when the shot peening is applied at plural steps for a long period of time, it is necessary to finish each distal end of the valve springs with a round surface to prevent breakage of the valve spring caused by defacement at its distal ends.